Hydrocarbon-producing wells often are stimulated by hydraulic fracturing operations, wherein a fracturing fluid may be introduced into a portion of a subterranean formation penetrated by a wellbore at a hydraulic pressure sufficient to create or enhance at least one fracture therein. Stimulating or treating the wellbore in such ways increases hydrocarbon production from the well. Fracturing equipment may be incorporated within a casing string used in the overall production process. Alternatively a casing string comprising fracturing equipment may be removably placed in the wellbore during and/or after completion operations. The casing string and fracturing equipment may be run into the wellbore to a predetermined depth. Various “zones” in the subterranean formation may be isolated via the operation of one or more packers, which may also help to secure the casing string and fracturing equipment in place.
Following placement of the casing string and fracturing equipment within the wellbore, it may be desirable to “pressure test” the casing string and fracturing equipment to ensure the integrity of both, for example, to ensure that a hole or leak has not developed during placement of the casing string and fracturing equipment. Pressure-testing generally involves pumping a fluid into the axial flowbore of the casing string such that a pressure is internally applied to the casing string and the fracturing equipment and maintaining that hydraulic pressure for sufficient period of time to ensure that a hole or leak has not developed. To accomplish this, no fluid pathway out of the casing string can be open, for example, all ports or windows of the fracturing equipment, as well as any additional routes of fluid communication, must be closed or restricted.
After a first pressure test has been performed and the integrity of the casing string and fracturing equipment has been confirmed, surface equipment may be removed and a period of time, sometimes several weeks or more, may pass. The well may be left unattended during this period of time. When ready to initiate a fracturing operation, the operator may often wish to perform a second pressure test to ensure that the integrity of the casing or fracturing equipment has not been compromised.
After the second pressure test, fracturing operations may commence. Such operations will require that a route of fluid communication out of the casing string and/or fracturing equipment be provided, either for the purpose of communicating fluid to the subterranean formation or circulating a device so as to actuate the fracturing equipment.
Conventionally, differential valves have been employed to provide a fluid pathway out of the casing string after a pressure test. Such differential valves are designed to open after a threshold pressure is reached. However, differential valves are often inaccurate as to the pressure at which they will open. Further, once a differential valve has been opened, it cannot be closed. Therefore, differential valves only allow for one pressure test at the threshold pressure. If a second pressure test is desired, either an obturating means (e.g., a dart or ball) must be employed to block of the fluid pathway via the differential valve or the first pressure test cannot reach a pressure at or approaching the threshold pressure at which the differential valve will open. Further still, once a pressure test has been performed at or near the threshold pressure, the well will be open, making it difficult if not impossible to achieve wellbore control following the first pressure test and thereby posing various risks, for example blow-outs or the loss of hydrocarbons. Therefore, there is a need for a tool which would provide a fluid pathway following the final of multiple pressure tests while maintaining wellbore control prior to completion of the final pressure test.